<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style lang="less">
    html {
        height: 100%;
    }
    
    body {
        height: 100%;
    }
    
    .parent {
        height: 100%;
    }
    
    .div3 {
        display: flex;
        /* vertical-align: center; */
        align-items: center;
        justify-content: space-around;
        height: 50%;
        background-color: pink;
    }
</style>

<body>
    <div class="parent">
        <div>111</div>
        <div>222</div>
        <div class="div3">
            <span>333</span>
        </div>
    </div>
</body>

</html>